ZSM-5 and its synthesis using tetrapropylammonium (TPA) cations as a directing agent are disclosed in U.S. Pat. No. 3,702,886. EP-A-21674 teaches that large crystal ZSM-5, having a crystal size in excess of 1 micron, can be prepared from a reaction mixture containing TPA cations provided the OH-/SiO.sub.2 ratio is maintained within the range 0.01-0.07.
Large crystal ZSM-5 (&gt;0.5 micron) usually can be prepared without difficulty when the aluminum content in the synthesis mixture is low. As a result, with large crystal ZSM-5, although SiO.sub.2 /Al.sub.2 O.sub.3 ratios of 100 or higher are relatively easily attainable, SiO.sub.2 /Al.sub.2 O.sub.3 ratios of 40 or lower are difficult to attain.
In conventionally prepared, large ZSM-5 crystals, the aluminum distribution tends to be non-uniform with a significantly higher aluminum concentration (&gt;20%) being found on the outer surface, as compared with the interior, of the crystal. Aluminum zoning in ZSM-5, especially in large crystals, has been reported. See "Zoned Aluminum Distribution in Synthetic Zeolite ZSM-5", R. von Ballmoos & W. M. Meier, Nature 1981, 289, 782-3; "Aluminum Distribution in Large ZSM-5 Crystals", K. J. Chao & J. Y. Chern, Zeolites, 1988, 8, 82; "Aluminum Zoning in ZSM-5 As Revealed by Selective Silica Removal", R. M. Dessau, E. W. Valyocsik & N. H. Goeke, Zeolites, 1992, 12, 776.